Stavros

Personally I would use Top stop available from Halfrords this is a lightweight car body filler which in my opinion would be the right stuff for the job.This stuff believe you me is so so easy to sand down and gives a superb finish as well,As Ken has allready said sealing and waterproofing come later as the paint you will apply will waterproof the hull.One little top tip I personally would do seeing as it is a wooden hull is to seal the hull prior to applying the Red oxide antifouling,this I would do by applying a couple of coats of Halfords car Lacquer before applying the primer,this will seal the wood and will do a far superior job than sanding sealer.You can then apply your primer on top of this and then your final colour.